Over about the last ten years, bicycles with frames fabricated from aluminum tubing have become increasingly popular. Unlike steel, aluminum cannot be brazed, so the joints between the tubes of most aluminum bicycle frames have been welded. The joints between the members of the main triangle usually involve welding an edge of a wall of one member, ordinarily an edge at the end of the member, to the surface of the wall of another member. In particular, the ends of the top tube and down tube are welded to the external surface of the head tube, and often to each other; the ends of the seat tube and the down tube are welded to the external surface of the bottom bracket tube and to each other; and the end of the top tube is welded to the surface of the seat tube. Each of these joints requires, first, forming the end of one tube so that it closely matches the surface of the other tube at a prescribed angle and, second, positioning the edge of that tube at the proper position axially and circumferentially as well with respect to the surface of the other tube.
The first of those steps has been performed using hole saws and special fixtures to set up a drill press to position the tube for forming the edge cut. For example, the down tube has a certain diameter and forms a certain angle with the head tube. To make the end cut in the down tube for the welded joint with the head tube, a hole saw having a diameter equal to the diameter of the head tube is installed in a drill press, and the down tube is held in a fixture so that the axis of the hole saw is at the same angle to the down tube as the head tube is in the completed frame. If the other end of the down tube has already been formed with an edge to match the bottom bracket tube, then the fixture also has to provide for making the cut at the proper axial location in the down tube stock with the tube in the proper circumferential position. The second step of the process requires all of the members of the frame to be set up in a special jig that will hold them in their proper relative positions while tack welds are made to establish the frame geometry.
All manufacturers usually make any given model of bicycle in several sizes. Each size requires that the members be of different lengths and that the cuts at the ends of the tubes be at different angles. That, in turn, requires making a different set-up of the drill press for each different size of any given frame member and for each different frame member. Because the edge cuts are almost always different for different size frames, as well as being different for different members of the frame, numerous fixtures and frequent changes in set-ups of the drill press are required to make the tubes for a given style of bicycle in a range of sizes. Similarly, a different erection jig is required for each size of bicycle. Accordingly, many manufacturers compromise the geometric variations required by different sizes in order to minimize production costs. When the manufacturer produces several models of bicycle, the amount of tooling and the down time for changing over from production of one size to another size frame, or from one style to another, are extremely costly. Also, if the manufacturer changes the design of a frame, considerable time and a large investment in tooling are required to make the change.